Vibrio cholerae is known to acquire iron in vitro from heme, hemoglobin, ferric citrate, and via siderophore, but the actual iron sources that are used during infection have not been identified. We hypothesize that alternate mechanisms of iron acquisition may be more relevant in vivo. Accordingly, genes necessary for utilization of ferric iron complexed with human transferrin as a sole iron source will be identify homologs of known genes required for TF- iron utilization in other species. Alternatively, mutant strains will be obtained following chemical or TnphoA mutagenesis and nalidixic acid or streptonigrin enrichment that are unable to use TF- bound iron. A gene(s) encoding a periplasmic or outer membrane protein(s) required for TF-iron acquisition will be cloned by complementation, the DNA and deduced protein sequences will be obtained, and a defined non-polar mutation will be created. An infant mouse model of cholera will be used to test for decreased virulence and/or colonization of mutants and for expression of the wild-type gene in vivo. Gene products involved in iron uptake and that are expressed in vivo could be useful targets for vaccine development.